In the existing cellular system, an array of antennas in a base station is typically arranged horizontally as illustrated in FIG. 1A and FIG. 1B. A beam of a transmitter in the base station can only be adjusted in the horizontal direction. With the development of the antenna technologies, an array of active antennas in which each antenna array element can be controlled separately has emerged in the industry as illustrated in FIG. 2A and FIG. 2B. With this array of antennas, it becomes possible to dynamically adjust the beam in the vertical direction.
In this three-dimension array of antennas, a signal transmitted by the base station can be used for beam-forming on a User Equipment (UE) in both the horizontal direction and the vertical direction. In order to obtain channel state information after beam-forming in the two dimensions, the base station typically performs beam-forming in the vertical dimension on a Channel State Information Reference Signal (CSI-RS) (as illustrated in FIG. 3), so that the UE feeds back Channel State Information (CSI) based upon the vertically beam-formed CSI-RS. In this way, the base station can pre-code data in the two dimensions including the vertical and horizontal dimensions, and perform link adaption, using a beam-forming vector in the vertical dimension for the CSI-RS, and the CSI fed back by the UE.
In a Full Dimension Multiple Input Multiple Output (FD MIMO) system, a cell may be configured with a plurality of CSI-RS resources, and different beam-forming vectors in the vertical dimension are applied to different resources. Since the CSI-RS is currently transmitted periodically, the UE needs to measure the CSI-RS in each period, and in order to feed back CSI periodically, the UE needs to report the CSI periodically, thus resulting in a high feedback frequency and a significant feedback overhead, which may degrade the spectrum efficiency.